Is sea level rising?
Global mean sea level has been rising at an average rate of 1 to 2 mm/year over the past 100 years, which is significantly larger than the rate averaged over the last thousand years. Projected increase for the 21st century is about 0.5 meter, but estimates range widely.
Can the observed changes be explained by natural variability, including changes in solar output?
Some changes, particularly part of the pre-1960 temperature record, show some relationship with solar output, but the more recent warm era is not well correlated. The exact magnitude of purely natural global mean temperature variance is not known precisely, but model experiments excluding solar variation indicate that it is likely less than the variability observed during this century.
Global Warming or Global Cooling the Threat for the Future?
Has the climate of the United States changed significantly during the century that is about to end? In what ways and by how much? Have national trends emerged that agree--or perhaps disagree--with what is expected from projections of global greenhouse warming? These are questions addressed in a report entitled "Trends in U.S. Climate during the Twentieth Century," by Thomas R. Karl, Richard W. Knight, David R. Easterling, Robert G. Quayle who serve on the scientific staff of the National Oceanic and Atmospheric Administration's National Climatic Data Center (NCDC), in Asheville, North Carolina. Thomas "The challenge to the climatologist is to separate any meaningful signals from ever-present noise, and to discern, if possible, whether there is indeed at work the sometimes slow and subtle hand of significant change. The second task, which is even harder, is to identify, unequivocally, the cause," according to the scientists was the focus of their study.
"Before such questions can be answered, we need to remind ourselves that 'climate', as it is defined for a specific region and time, includes more than the simple average of weather conditions. Either random events or long-term persistent change, or more often combinations of them, can bring about significant swings in a variety of climate indicators from one time period to the next. Examples include a year dominated by severe drought and the next excessively wet; a series of bitterly cold winters followed by winters more mild; one scorching summer preceded by a summer pleasantly warm; years with numerous severe storms followed by years with few severe storms. The temptation at each time and place is often to attribute any of these temporal and sometimes local variations to a wider and more pervasive change in climate ."
In their assessment they noted that the so-called "greenhouse" gases "have all been markedly increasing in amount since about the time of the industrial revolution, that began in earnest some 150 years ago. The largest and best-known contributor is carbon dioxide, originating principally from the burning of wood and coal and petroleum derivatives. However, other climatic trends include "changes in the composition of the atmosphere in ways that act to cool the surface temperature. This includes the anthropogenic decrease of stratospheric ozone, and an increase in anthropogenic microscopic sulfate particles, often readily apparent during the warm season as smog. The effect of these additional atmospheric constituents on global climate is less certain than that of the better known greenhouse gases, but models suggest that in some areas they may have already acted to significantly retard greenhouse warming. It is important to note, however, that the global-scale warming predicted in climate modeling experiments from future greenhouse gas increases is substantially larger on a global average than the regional cooling expected from these other sources.
Measurements of past and current levels of carbon dioxide and other greenhouse gases indicate that we should have already increased the global greenhouse effect by man-made, or anthropogenic additions, by nearly 40% in the last 150 years. If these changes were the only process of importance, then the same mathematical climate models suggest that the average global surface temperature should have risen by about 1° C during this time. Available climate data suggest that the mean global temperature has indeed risen, but unsteadily and by only about half that amount.
"Confounding any search for anthropogenic effects are the natural changes and variations of climate that will constantly add to or subtract from the expected signal. Examples include changes in upper atmospheric steering winds (commonly known as the jet stream) due to ocean-atmosphere interactions; changes in the circulation of the ocean that can influence air temperatures; effects of major volcanic eruptions; feedbacks from changes in the land surface, as in soil moisture, snow cover, and plant cover; and changes in the energy received from the Sun.
Реферат опубликован: 11/04/2010